Various techniques have been described for producing shades or gray tones by ink jet printing. U.S. Pat. No. 3,604,846 granted to D. Behane et al. on Sept. 14, 1970 discloses a technique wherein a matrix of nine cells is formed with each cell being of the maximum intensity of the ink jet mechanism. As described by Behane, lighter gray shades are formed by recording or printing only some of the cells within the matrix area. The greater the number of cells printed in the given matrix area, the darker is the reproduction with all cells having the same size intensity. However, this approach introduces certain undesirable results in that a matrix having equal density dots selectively applied generally exhibits a relatively coarse visual texture. If the size of the matrix cell area are made large enough to contain a sufficient number of dots to accommodate a large range of gray tones, the reproduction itself lacks detail and is rather coarse in appearance.
Still another system is taught by R. G. Sweet in U.S. Pat. No. 3,373,437, granted Mar. 12, 1968. This reference discloses the formation of gray shades by depositing a different number of ink drops at various locations thereby varying the size of the printed dot and thus the darkness or tonal quality of the selected location. The use of multiple drops of ink deposited at one position to produce a black dot naturally leads to a half tone technique. The production of gray tones is accomplished by reducing the number of drops at each dot position in the lighter tones. However, to produce an acceptable distribution of tones at the lighter end of the tonal scale requires an excessive number of drops for each position. Thus, a particular limitation with the system taught by Sweet is that the number of drops required to provide a satisfactory tonal scale may be prohibitively large to achieve a maximum degree of darkness while still permitting sufficient gradations at the lighter tones.
A further arrangement for generating tonal gradations is described in U.S. Pat. No. 3,977,007 issued Aug. 24, 1976 entitled "Generating Gray Tones in Ink Jet Printers" by Berry et al. and having a common assignee with this application. This reference discloses a device wherein gray shades are generated by depositing one drop at each location with lighter tones created by leaving some of the positions blank. Darker tones are created by depositing two or more drops at some or all of the positions. Thus, a matrix is utilized and once all of the matrix cells have received one drop of ink, darker shades are produced by increasing by one drop selected cells in the matrix following a predetermined pattern.
The problems encountered in the generation of gray tones in a digital system which provides only a finite number of discrete tones may be considered from two aspects; firstly, the number of tomes to be used and secondly their distribution within the gray scale. Prior systems have often placed an inordinate amount of emphasis on the number of available tones rather than their distribution within the gray scale. Actually, the important consideration should be the number of visually equal increments available. Thus, the limiting factor in a facsimile system having a finite number of tones is the largest increment between any two of the tones. An excellent reproduction can be had with as few as fifteen tones provided they are distributed equally in terms of visual perception. Equal visual increments are expressed in terms of the Munsell Value scale, which bears a direct, but not linear, relation to the physical measurable optical density.
A second measure of quality in facsimile systems is resolution which is often oversimply equated to the number of scan lines per inch. For conveying information, high resolution is required only for the large density changes that is most notably from white to black. At the smaller density steps linear progression of gray shades will not result in a linear progression of drops at selected dot locations. The change from a complete lack of drops to one drop or from a completely blank location to one drop at a selected location produces pronounced visual changes in the perception of the gray shade. The change from one less than the maximum number of drops at a location to the maximum number of drops capable of being placed at that location produces a much smaller visual effect than at the lighter end of the total scale. In other words, the sensitivity of the human eye to various shades of gray follows a percentage of logarithmic relation rather than a linear curve. Thus, it will be appreciated that in each of the well-known systems, significant fewer tones are possible than the maximum number of drops at a selected location would at first suggest.
The illustrated embodiment is particularly adapted for generating tonal variations at the low end of the intensity scale and provides a far greater number of tonal variations within the low end of the range than has heretofore been possible. The device generates half drop intensities over a given area by what may be termed an averaging technique. Selected dot locations printed in response to the same tonal intensity may vary by one drop in a random manner so that the average number of drops over a sequence of dots in response to a single tonal value will average fractionally.
It is a main object of the present invention to produce a relatively large range of gray tones with an ink jet printer. It is still a further object to provide such a printer which produces a large number of variations at the lighter end of the tonal scale. Other objects, advantages, and features of the invention will be readily appreciated with reference to the following description of the accompanying drawings.